Field of the Invention
The present invention concerns improvements to oxygen gauges. The principle of oxygen gauges, which is well known to the man skilled in the art, is essentially based on the use of a reference electrode, an ion conductive electrolyte and a working electrode sensitive to the oxygen partial pressure.
The best known gauges or sensors use zirconium dioxide (ZrO.sub.2) doped with yttrium oxide (Y.sub.2 O.sub.3) or calcium oxide (CaO). The operating temperature of the cell is more than 400.degree. C. due to the low conductivity at low temperature of this solid solution. The utilisation of other solid solutions based on cerium oxide (CeO.sub.2) or bismuth oxide (Bi.sub.2 O.sub.3), has been envisaged. However, at a temperature lower than 400.degree. C., the response time of the sensor (i.e. the time necessary for the e.m.f. of the sensor to become, after modification of the oxygen pressure of the gas, equals approximately 50% (t.sub.50) or 90% (t.sub.90) of the theoretical e.m.f.) becomes very high. On the other hand in the presence of a reducing gas, the electronic conductivity of these oxides is important and therefore the use of such electrolytes can only be envisaged for the measure of oxygen contents higher than 1%.
To remedy these difficulties a solid electrolyte has been used, the conductivity of which is mainly due to ions other than the oxide ions. Anion conducting solid electrolytes (chlorides, fluorides) synthesized in the past few years show sufficient conductivity at room temperature. The use of a lead fluostannate PbSnF.sub.4 has been proposed in French published Patent Application No. 2 486 244. Another French Patent Application, FP-ANo 82 13473 the title of which is "Electrochemical device for the measurement of partial pressure of oxygen in a gaseous or liquid atmosphere", proposes to dissolve in the lead fluostannate, peroxide ions for example 0.5% of barium peroxide (BaO.sub.2). Response time of the sensor has in this way been considerably reduced. However, operating temperature of the sensor, cannot be lower than 150.degree. C. due to the prohibitive increase in the response time of the sensor.
The object of the present invention is to realize oxygen gauges able to be used at temperatures below 150.degree. C., specially below 100.degree. C. and leading to excellent response times.
The present invention thus proposes an oxygen gauge of the type mentioned herein above in which the working electrode is constituted, at least partly, by a macrocyclic tetrapyrolic material.
The macrocyclic material can be chosen from the porphyrines shown in formula I: ##STR1##
In this formula:
the groups B.sub.1 to B.sub.8, which can be identical or different, each represent H or a methyl or ethyl radical. PA1 A represents either nitrogen in which case tetra-azoporphyrines are involved or represent C-H or a C-phenyl group.
Specially adapted tetra-azoporphyrines for the carrying out of the present invention are represented by formula II: ##STR2##
These derivatives correspond to the derivatives of formula I wherein the position of two adjacent radicals B.sub.1 to B.sub.8 are occupied by benzenic cycles condensed on the pyrolic cycle.
Among these derivatives, particularly the ones obtained by substituting the protons by a halogen (chlorine or fluorine) or by condensation of benzenic cycles (naphthophthalocyanine) will be used.
Preferably the macrocyclic material is a metallic complex, chosen among complexes of iron, nickel or cobalt.
The advantage of the working electrode according to the present invention is due to the fact that the macrocyclic tetra pyrolic materials already show at room temperature excellent catalyzing properties with respect to oxygen reduction;
In the case of metallic complexes, the electronic structure of the central atom is more particularly an important parameter for the catalyzing properties.
According to another embodiment, the electrolyte is a lead fluostannate doped with barium peroxide in which the peroxide is dissolved at 0.5%. This type of electrolyte is described in French patent application No. 82 13473.
According to another characteristic, the material of the reference electrode is a mixture of tin and of tin fluoride in contact with the electrolyte and isolated from atmosphere by an impervious resin.
According to the invention, the electrode material can be synthesized by all known methods. It is possible for example to synthesize the phthalocyanines from a phthalonitrile and a metallic salt, or from phthalonitrile and a metal-carbonyl in a solvent having a high boiling point.